Project Report of Electricity Billing System Page 1

Project Report of Electricity Billing System 
 
Page - 31 
There are seven rules for construct a data flow diagram. 
i) 
Arrows should not cross each other. 
ii) 
Squares, circles and files must wears names. 
iii) 
Decomposed data flows must be balanced. 
iv) 
No two data flows, squares or circles can be the same names. 
v) 
Draw all data flows around the outside of the diagram. 
vi) 
Choose meaningful names for data flows, processes & data stores. 
vii) 
Control information such as record units, password and validation 
requirements are not penitent to a data flow diagram. 
Additionally, a 
DFD
can be utilized to visualize data processing or a structured design. 
This basic DFD can be then disintegrated to a lower level diagram demonstrating 
smaller steps exhibiting details of the system that is being modeled. 
On a DFD, data items flow from an external data source or an internal data store to an 
internal data store or an external data sink, via an internal process. It is common 
practice to draw a 
context-level data flow diagram
first, which shows the interaction 
between the system and external agents, which act as data sources and data sinks. On 
the context diagram (also known as the Level 0 DFD’), the system's interactions with the 
outside world are modeled purely in terms of data flows across the system boundary. 
The context diagram shows the entire system as a single process, and gives no clues 
as to its internal organization. 
This context-level DFD is next "exploded", to produce a Level 1 DFD that shows some 
of the detail of the system being modeled. The Level 1 DFD shows how the system is 
divided into sub-systems (processes), each of which deals with one or more of the 
data flows to or from an external agent, and which together provide all of the 
functionality of the system as a whole. The level 1 DFD is further spreaded and split 
into more descriptive and detailed description about the project as level 2 DFD.The 
level 2 DFD can be a number of data flows which will finally show the entire 
description of the software project. 

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